Heat rash generally clears after cooling the skin. The AAP specifically advises against thick, greasy ointments and recommends cooling and carefully drying skin folds such as the armpits (
AAP advice). The medical text also describes miliaria as self-limited and usually resolving quickly in a cooler environment (Textbook of Family Medicine 9e, skin section).
See a pediatric clinician promptly if there is fever, increasing redness/swelling, pain, pus, blisters, spreading rash, or the child seems unwell. If it does not improve within 2 to 3 days of keeping cool and dry, have it checked because armpit rashes can also be eczema, fungal irritation, or infection.